The Hg.sub.1-x Cd.sub.x Te semiconductor material is vital to the research and the manufacture of the far-infrared photo-detector in view of the fact that the bandgap of this material ranges between 0.11 and 0.25 eV when x=0.2-0.3 and under the circumstance of 77K. This material is especially suitable for making a far-infrared photo-detector for the wavelength sections of 3.about.5 .mu.m and 8.about.12 .mu.m. However, the surface of this ternary semiconductor material has a high chemical reactivity as well as the unstable chemical and electrical structures. As a result, an excellent surface passivation is called for in the manufacture of the IR element capable of excellent performances.
The excellent surface passivation must meet the following requirements:
(1) INTERFACE PROPERTIES: PA1 (2) DIELECTRIC PROPERTIES:
a. high breakdown electric field PA2 b. small leakage current PA2 c. low interface energy state density PA2 d. low surface recombination velocity PA2 a. good insulator PA2 b. excellent adhesion PA2 c. thermal stability PA2 d. exhibition of radiation hardening
So far as the above requirements are concerned, a number of research papers dealing with the surface passivation layers of the HgCdTe material have been disclosed so far, as exemplified by CdS, CdTe, ZnS, anodic oxide, anodic sulfide, anodic fluoride, plasma oxide and SiO.sub.2 [B. K. Janousek, and R. C. Carscallen, J. Vac. Sci. Technol. A3, p 195, (1985); J. A. Wilson and V. A. Cotton, J. Vac. Sci. Technol. A3, p 199, (1985); J. F. Wager and D. R. Rhiger, J. Vac. Sci. Technol. A3, p 212, (1985); B. K. Janousek, R. C. Carscallen, and P. A. Bertrand, J. Vac. Sci. Technol. A1, p1723, (1983); J. D. Lin, Y. K. Su, S. J. Chang, M. Yokoyama, and F. Y. Juang, J. Vac. Sci. Technol. A12, p 7 (1994).]
In addition to the surface passivation, other important aspects of the subject matter include the stable state of the homogeneous stoichiometric composition of the surface of the HgCdTe material, the cryogenic deposition technique, and the pre-deposition treatment of the HgCdTe material surface. In order to avert the damaging effect of an excessively high temperature on the surface of the HgCdTe material, the electrochemical method was generally employed for forming the anodic oxide films in the early research. The films formed by the electrochemical method are poor in interface properties, thermal stability and repeatability. Moreover, the electrochemical method must be carried out at a treatment temperature of 60.degree. C. or higher and is therefore incapable of forming a good surface passivation.
In view of the drawbacks of the electrochemical method described above, the chemical vapor deposition (called CVD for short) was introduced for forming the silicon dioxide (SiO.sub.2) film on the HgCdTe substrate. The HgCdTe material is rather sensitive to temperature and is therefore very vulnerable to damage caused by the high temperature. In other words, the chemical structure of the HgCdTe surface can be easily destroyed by a temperature in excess of 100.degree. C. On the other hand, if the temperature is too low, the HgCdTe surface is devoid of sufficient energy to decompose the gases of SiH.sub.4 and oxygen in CVD method. As a result, the photo-CVD method must be used in place of the CVD method. The photo-CVD method makes use of the Hg and the D.sub.2 lamps for generating a strong radiation energy in the spectrum range of violet radiation and ultraviolet radiation. The radiation energy is used as a catalyst for accelerating the process of decomposing the SiH.sub.4 gas and the oxygen gas. However, the growth of the silicon dioxide film on the HgCdTe substrate by the photo-CVD method takes place at an undesired temperature ranging between 60.degree. C. and 100.degree. C. In addition, the photo-CVD method needs an expensive vacuum equipment.